Optical nonliearity of the liquid suspension of dielectric spheres has been studied with a single beam technique. Nonlinear refractive index n2 of polystyrene latex suspension with 0.059 μm-dimeter and concentration of 2.5% has been measured to be 4.3 ×10-9 cm2/W. The contribution of thermal effect has been proved experimentally to be negligible as compared with that of electrostrictive effect.